The present application relates to engine control systems and more particularly, but not exclusively, to engine control systems for controlling knock within spark-ignition gasoline engines.
Internal combustion engines can generally be grouped into one of two classes: spark-ignition and compression-ignition engines. Spark-ignition engines typically operate by introducing a stoichiometric mixture of air and fuel into a cylinder of an engine. A piston then compresses this mixture, and at a predetermined crankshaft angle, a spark plug will ignite the air/fuel mixture producing a flame front that propagates through the combustion chamber. The rapid increase in heat from the burned fuel triggers an increase in pressure which forces the piston downward in the cylinder. The ability to precisely time the combustion event through the use of a spark plug is a benefit of the spark-ignition engine. However, the spark-ignition engine may be somewhat inefficient since the compression ratio of the engine is kept to a relatively low level to avoid “knock.” Knock occurs when the air/fuel mixture ignites independently of the spark plug and may cause engine damage. Consequently, spark-ignition engines typically have compression ratios in a range of 8 to 11.
The compression-ignition engine, on the other hand, operates at relatively high compression ratio that is typically within the range of 15 to 22. This high compression ratio boosts the mechanical efficiency of the compression-ignition engine. The compression-ignition engine operates by introducing unthrottled air into the cylinder to increase the efficiency over that of the throttled spark-ignition engine by decreasing pumping losses. In a traditional compression-ignition engine, ignition timing is controlled by the injection of diesel fuel into the cylinder near the end of the compression stroke, when the trapped air within the combustion chamber is of a sufficient temperature to ignite the fuel. The heat released during the combustion process causes an increase in in-cylinder pressure which then forces the piston downward in the same fashion as the spark-ignition engine.
While more efficient than spark-ignition engines, compression-ignition engines produce more of certain types of emissions which often require expensive aftertreatment. Consequently, it would be desirable to improve spark-ignition engines so as to control, minimize or otherwise prevent the occurrence of knock therein.